1. Field of the Invention
The present invention relates to an optical frequency stabilizer and a method for stabilizing an optical frequency applicable to wavelength division multiplexing (WDM) transmission.
2. Description of the Related Art
As generally known, a laser diode (LD) used for WDM transmission system outputs a forward laser beam and a backward laser beam. The forward laser beam output from the laser diode is input to an optical fiber which is connected to transmission path. The backward laser beam output from the laser diode is input to a conventional optical frequency stabilizer.
The stabilizer comprises a Fabry-Perot resonator formed by an etalon. In the stabilizer, the laser beam is input to the etalon. The laser beam transmitted through the etalon is converted to an electrical signal. The electrical signal is compared with a reference value, obtaining an error signal. The error signal controls the oscillation wavelength of the laser diode.
According to such a configuration, the optical frequency of the laser beam generated from the laser diode can be stabilized to a specified value of optical transmittance of the etalon. With regard to this technology, details appear in following document 1.
Document 1
K. Tatsuno et al., “50-GHz spacing, multi-wavelength tunable locker integrated in a transmitter module with a monolithic-modulator and a DFB-laser”, in Technical Digest OFC (Optical Fiber Communication Conference and Exhibit) 2001, USA, TuB5.
The etalon can be made of quartz glass, quartz crystal or the like. A quartz crystal etalon is hardly influenced by external temperature in comparison with an ordinary etalon, because the temperature coefficient of the quartz crystal etalon is as small as 500 MHz/deg. Even if the stabilizer has a quartz crystal etalon, the optical frequency varies by several 100 MHz when the external temperature varies by 40 deg. The temperature influence described above is a considerable problem to the laser diode used for the WDM transmission system, because the system requires optical frequency in the order of one GHz.